The present invention relates to manual air displacement pipettes and more particularly to an improved manual pipette including a moveable plunger and a mechanical assist for aiding a pipette user in manually locating and maintaining the plunger at a xe2x80x9chomexe2x80x9d position ready to aspirate a predetermined volume of liquid.
U.S. Pat. Nos. 3,827,305 and 4,909,991, for example, describe commercially available single channel manual air displacement pipettes. Each such pipette includes an elongated hand-holdable pipette body housing an upwardly spring biased plunger unit. The plunger unit is supported for axial movement in the pipette body between a first or upper stop position in which an end portion of the plunger unit extends from an upper end of the pipette body. A pipette user grips the pipette body with his or her thumb over the exposed end of the plunger unit. Downward thumb action on the plunger unit moves the plunger unit downward from its upper stop position against the upward bias of a return spring to a second or a lower stop position at which all fluid is expelled from a tip secured to the pipette. Adjacent the lower stop position is a xe2x80x9chomexe2x80x9d position for the plunger unit to which the plunger unit is returned by the pipette user at the beginning of each aspiration operation with the pipette.
In the commercially available pipettes described in the foregoing patents, the home position is defined by a xe2x80x9csoftxe2x80x9d stop. As described in such patents, the soft stop comprises a second relatively stiff spring mechanism within the pipette body which is activated when the plunger unit reaches the home position. In this regard, and as depicted in FIG. 4a herein, as the pipette user manually moves the plunger unit from its upper stop position by pressing downwardly with his or her thumb on the exposed end of the plunger unit, the pipette user can xe2x80x9cfeelxe2x80x9d an increased resistance to movement of the plunger unit associated with an activation of the second spring assembly opposing further downward movement of the plunger unit. The position of the plunger unit where the user feels the activation of the second spring mechanism defines the home position for the plunger unit. Continued movement of the plunger unit beyond the home position to the lower stop position is resisted by a combination of the return spring and the second spring mechanism.
Thus, in pipeting liquids with such commercially available pipettes, the pipette user grasps the pipette housing with his or her thumb on top of the exposed end of the plunger unit. Exerting downward thumb pressure on the plunger unit, the user moves the plunger unit away from the upper stop position against the force of the return spring. The user detects the home position for the plunger unit during movement of the plunger unit away from the first stop position by sensing the start of an increase in the downward force required to move the plunger unit. Such increase force is the result of movement of the plunger unit against the return spring and the second spring mechanism, commonly referred to as a xe2x80x9cblowoutxe2x80x9d spring mechanism. Accurate sensing of the start of the increase in the downward force required to move the plunger unit is a delicate operation requiring great care to be exercised by the pipette user. Thus, with his or her thumb on top of the exposed end of the plunger unit, the user very carefully senses and then manually maintains the plunger unit at the home position. In practice, a significant portion of the total time associated with a pipeting operation is occupied by the pipette user manually maintaining the plunger unit at the home position ready for insertion of a tip extending from the pipette into the liquid which is to be aspirated by the pipette. Then, with the tip inserted in the liquid, the user manually controls the rate of return of the plunger unit from the home position to the upper stop position.
For accuracy and repeatability of operation of the pipette, it is important that the pipette user always bring the plunger unit to the exact same home position and that the pipette user manually control the rate of return of the plunger unit to the upper stop position in a repeatable manner for each pipette operation. This is necessary in order that the same desired volume of liquid will be drawn into the pipette tip during each repeated operation. It should be appreciated that such manual operation of a pipette places substantial physical and mental strain upon the pipette user over the course of a series of pipette operations wherein repeatability of operation is essential. In extreme cases, the physical hand and wrist strain associated with extensive and prolonged manual pipette operation can contribute to or produce repetitive strain injuries such as tendinitis and carpel tunnel syndrome.
Similar physical and mental stress problems are associated with other manual pipettes which include different mechanisms for defining the plunger unit home position. Examples of such different mechanisms are described in U.S. Pat. No. 4,041,764 and in German patent applications 239 539 A1 and 239 540 A1. Specifically, U.S. Pat. No. 4,041,764, describes a magnetic detent which is engaged between an upper stop and the pipette user exerting an increased axial force on a push button when it is desired to move the piston beyond the home position against the force of a return spring. The manual forces which a user of the pipette of U.S. Pat. No. 4,041,764 must exert on its pipette piston (plunger) in moving the piston from its upper stop position to and through a home position to a lower stop position are depicted in FIG. 4b herein.
The German patent applications, on the other hand, each describe a hollow piston pipette with ferromagnetic systems at upper and lower stops. The lower stop is a xe2x80x9chardxe2x80x9d bottom stop for the hollow piston in that no piston movement beyond the lower stop is permitted. A user of the hollow piston pipette does not have to xe2x80x9cfeelxe2x80x9d a xe2x80x9csoftxe2x80x9d stop defining a home position for the hollow piston. Rather, the lower stop defines the home position for the hollow piston pipette. Thus, in the operation of the hollow piston pipette, the user simply grasps the pipette body and by exerting a downward thumb force on an activating knob drives the hollow piston to the lower stop. To aspirate liquid into a tip connected to a lower cone of the hollow piston pipette, the user simply releases the activating knob and allows a compression spring to move the hollow piston from the lower stop to the upper stop. The ferromagnetic systems of the upper and lower stops interact with a magnetized locking piece to control operation of a disk seal in opening and closing the aperture of the hollow piston. For example, since the retaining force of the ferromagnetic system of the lower stop is greater than that of the locking piece and the axial motion of the locking piece is limited by a stop, the disk seal lifts away from a flange on the hollow piston and frees the aperture of the hollow piston so that a first cylinder-piston system communicates with a lumen of the pipette tip through the hollow piston and holes leading to a ventilation channel to atmosphere.
It is to be noted that in all of the foregoing manual pipettes, the pipette user is required to continuously apply steady downward force with his or her thumb to maintain the pipette plunger unit in its home position ready for insertion of a tip of the pipette into the liquid to be drawn into the tip by controlled upward movement of the plunger unit from the home position to its upper stop position.
Recognizing the physical and mental strain associated with repeated and prolonged operation of a manual pipette by a pipette user and to significantly reduce the physical and mental strain associated with the operation of manual pipettes and eliminate the need for the pipette user to physically maintain a pipette plunger in a home position, a latch mechanism operable as a pipette plunger reaches the home position has been recently developed and is described and illustrated in U.S. Pat. No. 5,364,596 assigned to the assignee of the present invention. As described in U.S. Pat. No. 5,364,596, the latch mechanism releasably maintains a plunger in the home position without any user exerted force on the plunger in opposition to the force of the return spring. Such an improved manual pipette may further include a velocity governor for automatically controlling the rate of return movement of the piston from the home position to the upper stop position for the plunger upon a release of the latch mechanism.
While such improved manual pipettes including latch and velocity governors improve the repeatability and reliability of operation of manual pipettes and reduce the physical and mental strain on pipette users where repeatability of operation is essential, they introduce significant increases in the manufacturing costs for manual pipettes which are reflected in increased prices for such improved manual pipette over their more simple predecessors.
More recently, an improved manual pipette which is of simple construction and low in manufacturing cost has been developed which provides a significant reduction in the physical and mental strain on a pipette user over the course of a series of pipette operations where repeatability of operation is essential. That manual pipette is described and illustrated in U.S. Pat. No. 5,700,959 assigned to the assignee of the present invention.
As previously stated, in prior conventional manual pipettes, the pipette user must exert a relatively strong downward thumb force on the plunger unit to retain it in the xe2x80x9chomexe2x80x9d position in opposition the return spring and a relatively strong xe2x80x9cblow outxe2x80x9d spring defining the xe2x80x9csoftxe2x80x9d stop. With the manual pipette described in the ""959 patent however, rather than requiring the user to carefully sense the exact start of a sudden increase in a force opposing downward movement of a plunger unit in locating the xe2x80x9chomexe2x80x9d position for the plunger unit and rather than requiring the user to manually exert a strong downward force to maintain the plunger unit in its xe2x80x9chomexe2x80x9d position against the return and blow out springs, the manual pipette of the ""959 patent includes a magnet assist mechanism. The magnet assist mechanism generates a downward magnetic force in opposition to the return spring force as the unit reaches and is at the home position. The magnetic force is less than the upward force generated by the return spring and does not latch the plunger unit at the home position as is the ""596 patent. Rather, the opposition force generated by the magnet assist is (i) reflected in a reduction in the downward force required to move the plunger unit as it approaches the home position to aid the pipette user in sensing the home position, and (ii) reduces the manual force that the pipette user must exert to maintain the plunger unit in the home position. The magnet assist thereby substantially reduces the physical and mental strain on the pipette user over the course of a series of pipette operations wherein repeatability of operation is essential.
While the magnet assist mechanism described in the ""959 patent is simple in construction, low in cost and significantly reduces the problems associated with conventional manual pipettes with respect to physical and mental strain, some pipette users have expressed the desire to be able to exercise complete manual control over the travel of the plunger particularly in the region of its axial travel where the magnet assist mechanism is operational to aide in locating and maintaining the pipette plunger at its xe2x80x9chome positionxe2x80x9d. The present invention not only satisfies that request but also provides a manual pipette which does not require or utilize a blow out spring or a latch mechanism to define the xe2x80x9chome positionxe2x80x9d for the plunger unit.
Like prior conventional manual pipettes, the present invention comprises a hand holdable pipette body having a return spring biased plunger unit supported therein for axial movement from a first or upper stop position. As with prior manual pipettes, a pipette user holding the pipette of the present invention presses on a plunger control knob to move the plunger unit downward from the first stop position against the upward force of the return spring to a second or lower stop position wherein all fluid contained in a pipette tip secured to the pipette body is expelled from the tip. The pipette user then allows the return spring to return the plunger to a xe2x80x9chomexe2x80x9d position adjacent the lower stop position. The xe2x80x9chomexe2x80x9d position is defined by a xe2x80x9csoftxe2x80x9d stop and is the starting position to which the plunger unit is returned for the start of each successive aspiration operation with the pipette. In prior conventional manual pipettes, the pipette user must exert a relatively strong downward thumb force on the plunger unit to retain it in the xe2x80x9chomexe2x80x9d position in opposition the return spring and a relatively strong xe2x80x9cblow outxe2x80x9d spring defining the xe2x80x9csoftxe2x80x9d stop. In particular, any downward movement of the plunger unit beyond the xe2x80x9chomexe2x80x9d position activates the xe2x80x9cblow outxe2x80x9d spring which generates a strong upward force in opposition to such downward movement of the plunger unit. The pipette user senses or xe2x80x9cfeelsxe2x80x9d the start of the increase in the return force which provides the user an indication that the plunger unit has reached and is at the xe2x80x9chomexe2x80x9d position.
The pipette of the present invention, however, does not include a blow out spring. Further, with the present invention, rather than requiring the user to carefully sense the exact start of a sudden increase in a force opposing downward movement of a plunger unit in locating the xe2x80x9chomexe2x80x9d position for the plunger unit and rather than requiring the user to manually exert a strong downward force to maintain the plunger unit in its xe2x80x9chomexe2x80x9d position against a return spring and a blow out spring, the pipette of the present invention includes a mechanical assist mechanism. As the plunger unit reaches and is at the home position, the mechanical assist mechanism generates a lateral force which is translated by the plunger into a mechanical force in opposition to the return spring force. The mechanical opposition force is less than the upward force generated by the return spring and is reflected in a relatively small change in the downward force required to move the plunger unit as it approaches the home position and aids the pipette user in sensing the home position. Further, the opposition force generated by the mechanical assist reduces the manual force that the pipette user must exert to maintain the plunger unit in the home position. The mechanical assist thereby substantially reduces the physical and mental strain on the pipette user over the course of a series of pipette operations wherein repeatability of operation is essential.
Still further, the absence of a blow out spring in the pipette of the present invention means that a major operating force associated with all prior commercially available air displacement pipettes is eliminated in the present invention. That is, the large user generated downward force required to effect xe2x80x9cblow outxe2x80x9d of all liquid from the tip of the pipette in opposition to the strong blow out spring common in commercial air displacement pipettes. Thus, using a simple relatively low cost construction which eliminates the blow out spring common to air displacement pipettes, the present invention significantly reduces the problems associated with conventional manual pipettes with respect to physical and mental strain with only a minor increase in manufacturing cost.